Continuous process for producing biodiesel fuel

ABSTRACT

A continuous process for producing biodiesel fuel with a conversion of triglycerides in biodiesel above 99.90% by weight in the transesterification step, which comprises the following steps: 
     a) providing a triglyceride source at a triglyceride concentration above 99.0% by weight; 
     b) subjecting triglycerides to a transesterification reaction with methanol, ethanol, or the mixtures thereof, at a molar ratio of triglycerides:alcohol 1:3-3.5, in the presence of 0.7 to 1.0% by weight of sodium or potassium hydroxide as catalyst, under ultrasonic cavitation conditions in an ultrasonic cavitation reactor with 2 to 8 serial cavitation cells, at a temperature of 45 to 60° C., at a pressure of 1.5 to 2 MPa (15-20 atm) and during a period of time of 15 to 30 seconds; 
     c) recovering the product of step b) and subjecting it to mechanical agitation operation, at a temperature of 45 to 60° C. for a maximum of 10 minutes, up to the completion of the transesterification reaction, thus obtaining a conversion of triglycerides in biodiesel above 99.90% by weight; 
     d) decanting and/or centrifuging the product resulting from step c) to remove glycerin; 
     e) purifying biodiesel obtained in step d) by exchange resins; and 
     f) removing by distillation methanol and water from the product of step e) and recovering the biodiesel fuel thus obtained.

FIELD OF THE INVENTION

The present invention relates to a continuous process for obtaining biodiesel fuel.

This semi-synthetic type of fuel is used in pure form or blended with diesel fuel (also known as gasoil) for the operation of diesel engines.

The advantages of biodiesel fuel, compared to its fossil equivalent, diesel, have been known for many years:

This fuel derives from renewable resources.

It can be blended with diesel fuel at any rate, without altering the normal operation of the engine.

Biodiesel reduces dioxide emissions to the atmosphere, which are the main cause of the greenhouse effect that produces global warming, as well as sulfur and carcinogenic aromatic component emissions.

Finally, biodiesel is biodegradable, i.e. it is beneficial to the environment, and can be perfectly blended with gasoil: it neither starts nor bursts easily because it has a high ignition point or temperature.

Biodiesel is obtained from new or used vegetable oils and animal fats.

Although biodiesel may be obtained from animal fats, the process of the present invention will specifically refer to the transformation of vegetable oil triglycerides into fatty acid methyl esters, which form biodiesel, and are suitable to be used as fuel in diesel engines.

This process comprises a transesterification step which involves a high conversion of triglycerides into fatty acid methyl esters under ultrasonic cavitation conditions in an ultrasonic cavitation tubular reactor with 2 to 8 serial cavitation cells.

BACKGROUND OF THE INVENTION

The triglycerides present in vegetable oils or animal fats are composed of glycerin (1, 2, 3-propanetriol) molecules esterified with saturated and unsaturated fatty acids, mainly with 16 to 20 carbon atoms.

Soybean, sunflower, rape, palm, and even cotton oils are examples of vegetable oils which contain these types of triglycerides.

Fuel, commonly known as biodiesel, is fuel for diesel engines generally prepared from vegetable oils containing the abovementioned triglycerides.

Furthermore, diesel fuel, produced from oil fractional distillation, is a portion of linear and branched hydrocarbons (paraffins and olefins) containing about 15 to 25 carbon atoms per molecule.

It has long been proved that diesel fuel can be partially or totally replaced by biodiesel.

From the chemical point of view, biodiesel fuel is composed by fatty acid methyl (and eventually ethyl) esters containing mainly 16 to 20 carbon atoms.

For such reason, in the last decades, industrial chemical processes have been developed for preparing biodiesel fuel by substitution transformation of triglycerides which contain vegetable oils into fatty acid methyl esters.

This chemical process is commonly known as transesterification, and consists in the chemical reaction shown in the following scheme:

wherein R represents a hydrocarbon remainder containing 15 to 20 carbon atoms.

Nowadays, this reaction is carried out in the presence of sodium or potassium hydroxide, as catalyst.

Even though there are many technologies for carrying out the transesterification reaction, all of them have technical drawbacks and economic disadvantages.

One of the most modern technologies for preparing biodiesel fuel uses ultrasonic cavitation as an efficient method for carrying out the transesterification reaction.

Ultrasonic waves have frequencies above 20,000 Hz, and are thus inaudible to human beings.

When a liquid is subject to ultrasonic waves, an alternating succession of sudden compressions and decompressions within the liquid occurs.

These sudden decompressions may cause small gas bubbles, which subsequently collapse.

The gas bubble formation phenomenon in the liquid, resulting from the creation of a low pressure area (below the liquid vapor pressure), is called cavitation, and when cavitation is the result of applying an ultrasonic wave train, the phenomenon is known as ultrasonic cavitation.

From the physical point of view, ultrasonic cavitation in a liquid produces, at the same time, a large amount of small bubbles which subsequently collapse. This physical phenomenon results, at liquid volume level, in a violent agitation and the subsequent mixture of the liquid.

When the liquid is a mixture of two reactants, such as triglyceride and methanol, and the catalytic conditions as well as the temperature and pressure conditions are the suitable ones, then the result of applying the ultrasonic cavitation phenomenon is the chemical reaction between both reactants at a very high reaction rate.

Thus, when using an ultrasonic cavitation reactor for producing biodiesel, the transesterification step time is drastically reduced, e.g. from minutes or hours in conventional (discontinuous- or batch-like) processes to seconds in the ultrasonic cavitation process.

This ultrasonic cavitation process has been applied to the continuous manufacture of biodiesel with outstanding success.

The known equipment, which can be implemented at industrial scale, basically consists of two tanks containing, respectively, the oil and the methanol blend with the catalyst (sodium or potassium hydroxide).

Both solutions are fed, at about the appropriate stoichiometric molar ratios (1 mole triglyceride per 3 moles methanol), into an ultrasonic cavitation tubular reactor, wherein pressure ranges from 1 to 3 atm and temperature ranges from about 66 to 70° C.

After a residence time of 5 to 30 seconds (depending on the flow rate and the power provided by the ultrasonic device), the flow leaving the reactor must be sent to an agitated-tank-type reactor, within a residence time of about 1 hour, which is the time needed to yield the final conversion of triglycerides of about 96-98%.

Afterwards, the glycerin/biodiesel blend is pumped to the train of glycerin separation, methanol recovery, and biodiesel wash and drying.

However, this known process has technical and economic drawbacks.

Firstly, the need of an agitated-tank-type reactor after the cavitation reactor, designed for a residence time of about 1 hour, in order to obtain an acceptable triglyceride conversion, implies large and expensive structures for biodiesel production at industrial scale.

Secondly, the need of a reactor downstream from the cavitation reactor for obtaining acceptable levels of final conversion in the transesterification reaction (96-98%), within a considerably high residence time (1 hour), indicates that the conversion of triglycerides in biodiesel within the cavitation reactor is far from being optimal.

These problems have been solved now by the process of the present invention, which combines a set of novel and inventive operative conditions for the transesterification reaction, with an ultrasonic cavitation reactor provided with many serial cavitation cells.

DESCRIPTION OF THE INVENTION

The European standard EN 14214 represents an international standard which describes biodiesel minimum technical requirements.

Biodiesel, as fuel, can be compared to diesel fuel, which is produced from oil, but which has the advantage of being a fuel obtainable from renewable resources, such as oilseed crops (soybean, corn, etc.)

Pure biodiesel, known as B100, consists of fatty acid methyl esters of 14/15 to 24 carbon atoms (palmitic, oleic, linoleic, etc.) derived from glycerin.

Biodiesel and diesel fuel blends are indicated with a “B” followed by a number which represents the biodiesel contained in the blend. E.g., B80 is 80% biodiesel blended with 20% diesel fuel.

The standard EN 14214 sets forth, among other requirements, that biodiesel shall have a minimum methyl ester content of 96.5% by weight and a maximum combined glycerin content (mono-, di- and triglycerides) of 1.2% by weight.

These requirements imply that the transesterification chemical reaction step is particularly important in the manufacture of biodiesel fuel, since a maximum conversion of triglycerides in biodiesel represents an optimal use of raw materials (oil and methanol), thus reducing the fuel price per ton of used vegetable material, and a considerably lower cost in the subsequent steps of separating free glycerin, combined glycerin, methanol, catalyst, and soap residues and other impurities.

As mentioned before, ultrasonic cavitation is a known process for producing biodiesel, but the operative conditions and the equipment used so far have not provided fully satisfactory results.

In fact, the known biodiesel preparation processes under ultrasonic cavitation conditions have been carried out under the following operative conditions:

Cavitation reactor temperature: 65 to 70° C.;

Cavitation reactor pressure: 1 to 3 atm;

Catalyst concentration: 0.6% by weight of sodium or potassium hydroxide;

Type of cavitation reactor: tubular, with one ultrasonic cavitation cell;

Residence time in the agitated-tank-type reactor for completing the transesterification reaction: 1 hour;

Chemical reaction stoichiometry: 1 mole triglyceride per 3 moles methanol (or light molar excess of methanol).

Under these operative conditions, a maximum triglyceride conversion of 96 to 98% by weight is obtained at the agitated-tank-type reactor outlet.

On the contrary, after thorough research, applicants could develop a process for preparing biodiesel by means of ultrasonic cavitation technology, which allows obtaining, in the transesterification step, a triglyceride conversion above 99.0% by weight, preferably 99.90% to 99.95%. This conversion value of triglycerides in biodiesel is obtained by allowing the chemical reaction initiated in the cavitation reactor to end in an agitated-tank-type reactor downstream from the cavitation reactor.

The operative conditions of the process of the present invention for preparing biodiesel, according to the preferred embodiment, are the following:

Reaction temperature in both reactors: 45 to 60° C.; preferably 50° C.;

Cavitation reactor pressure: 1.5-2 MPa (15 to 20 atm); preferably 16.6 atm (250 psi);

Catalyst concentration: 0.7 to 1% by weight of sodium or potassium hydroxide; preferably 0.95% sodium hydroxide;

Residence time in the cavitation reactor: 15 to 30 seconds;

Maximum residence time in the agitated-tank-type reactor for completing the transesterification reaction: 10 minutes;

Triglyceride concentration of the triglyceride source: above 99.0% by weight;

Chemical reaction stoichiometry: 1 mole triglyceride per 3-3.5 moles methanol or ethanol;

Type of cavitation reactor: tubular, with 2 to 8 serial ultrasonic cavitation cells.

During the cavitation process, as the product passes through the cavitators, the generated energy forms vapor bubbles of the product which then collapse. Within the bubbles, pressure reaches 40,000 psi and temperature reaches 10,000° C., because the phenomenon occurs at a molecular level since the outer temperature is just 3 to 5 degrees above the product inlet temperature and the pressure virtually does not vary.

Under these operative conditions, a triglyceride conversion above 99.0% by weight, preferably 99.90% to 99.95% by weight is obtained at the agitated-tank-type reactor outlet.

These triglyceride conversion values, which in practice represent a complete conversion of triglycerides in biodiesel, have been obtained neither by conventional technologies nor by the ultrasonic cavitation technology used hitherto.

Moreover, these results derive from the particular combination of the temperature and pressure operative conditions in the ultrasonic cavitation reactor, which are substantially different from the values used in the state of the art, and from the application of a multiple-cell cavitation reactor.

In particular, the reaction temperature decrease below the usual 65 to 70° C., to values ranging from 45 to 60° C., and preferably about 50° C., together with the pressure increase in the cavitation reactor above the usual 1 to 3 atm up to values ranging from 15 to 20 atm, together with the use of an ultrasonic cavitation tubular reactor of multiple serial cells, for obtaining such high results in the conversion of triglyceride (above 99.0% by weight) in biodiesel, are really surprising and unexpected results which cannot be easily explained nor justified from the technical point of view.

Therefore, the present invention relates to a continuous process for producing biodiesel fuel with a conversion of triglycerides in biodiesel above 99.90% by weight in the transesterification step, which comprises the following steps:

a) providing a triglyceride source at a triglyceride concentration above 99.0% by weight; b) subjecting triglycerides to a transesterification reaction with methanol, ethanol, or the mixtures thereof, at a molar ratio of triglycerides:alcohol 1:3-3.5, in the presence of 0.7 to 1.0% by weight of sodium or potassium hydroxide as catalyst, under ultrasonic cavitation conditions in an ultrasonic cavitation tubular reactor with 2 to 8 serial cavitation cells, at a temperature of 45 to 60° C., at a pressure of 1.5 to 2 MPa (15-20 atm) and during a period of time of 15 to 30 seconds. c) recovering the product of step b) and subjecting it to mechanical agitation operation, at a temperature of 45 to 60° C., for a maximum of 10 minutes, up to the completion of the transesterification reaction, thus obtaining a conversion of triglycerides in biodiesel above 99.90% by weight.

After step c), the product is decanted and/or centrifuged to remove glycerin, then biodiesel is purified, e.g. by exchange resins, and afterwards methanol and water are distilled to yield the pure biodiesel fuel.

Below is an experimental example explaining the details of the process, which is the subject-matter of the invention, applied to the particular and representative case of soybean oil.

Such example also includes an explanation of the fitting operations and processes of the raw material used (raw soybean oil) which is fed into the cavitation reactor, and of the known operations and processes for treating the product obtained from the agitated-tank-type reactor (RTA) up to the purified biodiesel fuel.

Application Example

In order to carry out the process of the invention, ultrasonic cavitation equipment, called F-9000, provided by Cavitation Technologies Inc., was used.

This equipment is composed of a tubular reactor provided with 7 serial ultrasonic cavitation cells.

A batch of 1000 L raw soybean oil was used as raw material, with a density of about 0.92 Kg/L.

In the transesterification step, sodium hydroxide was used as catalyst (0.9% by weight compared to the total amount of oil plus alcohol). Advantageously, this type of catalyst allows working under low temperature conditions, below the normal methanol boiling temperature. Technical grade methanol was used with purity above 98.5% by weight.

Since the free fatty acids contained in the starting oil are not esterified during the transesterification process, such compounds were previously transformed in their methyl esters before the transesterification step.

The final steps of the complete process, following the transesterification step, consisted in separating biodiesel fuel from the remaining byproducts, specially the residual methanol and glycerin.

Thus, the transesterification process was followed by glycerin separation, which is insoluble in methyl esters. Besides, methanol excess, catalyst residues, and soap generated as byproducts had to be removed, as well as the non-esterified free fatty acids.

In conventional processes, this is usually carried out with water, and then the clean methyl ester can be used directly or else be subject to a complete distillation process until a pure methyl ester is obtained, making the process more expensive in terms of energy.

The process of the present invention was performed without being necessary to use neither water nor acid for its process, which provided the possibility of using any raw material, and which allowed using an amount of methanol 30% less than the amount used in the batch-like conventional processes.

The complete process for obtaining biodiesel from soybean oil, in a preferred exemplary embodiment, comprised the following steps:

1.—Oil decantation and centrifugation. 2.—Degumming with B-19-like ion exchange resin column provided by the company DOW CHEMICALS. 3.—Transformation of oil acidity (caused by free fatty acids) in biodiesel using a catalyst, e.g. the B-20-type, provided by the company DOW CHEMICALS, and subsequent water removal by distillation. 4.—Transesterification by ultrasonic cavitation using an ultrasonic cavitator Force 9000, provided by Cavitation Technologies Inc., followed by a product agitation process in a tank during 10 minutes to complete the transesterification reaction. 5.—Glycerin separation by centrifugation, or decantation in columns and subsequent centrifugation. 6.—Biodiesel purification by B-10-like exchange resin columns. 7.—Biodiesel drying by flash distillation. Step 1: For step 1 of raw soybean oil decantation and centrifugation, a conical tank of appropriate volume was used to decant solids and water, and then a centrifuge was additionally used in order to obtain a homogenous raw material without solids and water. Step 2: The purpose of the subsequent degumming step was to remove impurities in the decanted and centrifuged oil, which are generally called “non-saponifiable” materials, and include proteins, phosphatides, lecithin, mucilage, sterols, and hydrocarbons.

This process was carried out by means of resins capable of removing these impurities at a reasonable cost. This resin technology, called AMBERSEP BD19, is a purification technology specially designed to be used with catalyst AMBERLYST BD20, a solid catalyst for the esterification of free fatty acids in raw material. This system is a cost-efficient solution for converting all free fatty acids into biodiesel.

It is noted that up to now, all raw material treatment processes for the transesterification have been carried out with acids.

With these types of resins and catalysts, called BD19 and BD20, respectively, provided by the company Dow Chemicals, a feed stream without residual acidity to the transesterification step was obtained, thus gaining more than 3% triglyceride conversion per 1% acidity contained in the raw material used.

Furthermore, the process was carried out without using acids and without water for the product wash, which is used for removing the soap generated during neutralization with conventional processes.

As regards the use of AMBERSEP BD19 technology, a temperature of 60° C. was applied (although it is possible to use a temperature of up to 75° C.), so that the oil reasonable viscosity would limit the pressure drop through the resin bed. It is preferred to apply column back pressure, of about 10 psi (0.69 bar), to reduce the negative impact of the air drawn into the column. It is advisable that the column functions at no more than 30 psi (2 bar) of differentiated pressure.

The amount of resin AMBERSEP BD19 used was 5,500 kg per 5000 monthly tons of biodiesel production, equivalent to 10 m³/h of biodiesel production. The column size had 30% empty space (7.17 m³) and a coupling filter of 80 microns. The building material was stainless steel 304L. The resin change frequency, depending on the impurities level, was of up to 30 days.

Step 3: In this step, oil acidity (from free fatty acids) was transformed in biodiesel using catalyst AMBERLYST BD20. These types of reactors are designed to function at a maximum temperature of 110° C., corresponding to about 55 psi (3.6 bars) pressure for pure methanol. The design pressure must be higher than these values to ensure that methanol remains in the liquid phase.

For normal operation, the temperature must range from 80 to 90° C. At higher temperatures, the temperature and the reaction increase, and the reactor bed BD/20 begins to increase in volume.

An acceptable conversion takes place at 65° C. using the fresh catalyst, thus representing a minimum specification of the design.

Furthermore, an evaporator was set to remove the water produced in this free acid esterification step. Since the methanol boiling point is lower than that of water, methanol was removed together with water. Methanol and water evaporation was achieved by reducing pressure (under vacuum conditions) and increasing the solution temperature.

At the end of this step, the water-in-oil content could be reduced by less than 500 ppm. Higher water levels would adversely affect conversion in the following transesterification step.

As regards the expected methanol consumption in this step, it is known that one mol of methanol per each mol of free acid is needed in the feed (centrifuged and degummed soybean oil).

For example, for 100 L/h oil volumetric flow with 10% by weight of free acid content, the total consumption of theoretical methanol would be 1.25 L/h.

An additional distillation step is also needed to separate methanol from the water generated in the esterification process using the catalyst BD20.

In relation to the acidity transformation process, the amount of resin Amberlyst BD20 needed to continue the process of the previous steps is 7,000 kg. The column size must have 10% empty space, 7.70 m³, using stainless steel 304L as building material. The change frequency depends on the impurities level and can be renewed every 180 days. An 80-120 metallic mesh at the bottom of each reactor is preferred.

Step 4: Transesterificacion. Surprisingly, it has been found that only under certain operation conditions in the ultrasonic cavitation reactor, not obvious for a person skilled in the art, conversions of 99.95% by weight of triglycerides have been achieved.

At the input of the ultrasonic cavitation process, in the Force 9000 equipment, the feed, composed of a volumetric flow of 16,000 L/h and 86% soybean oil with purity above 99.0% by weight and 14% by weight of methanol, was heated at 50° C. It is worth noting that satisfactory results can also be obtained by operating at temperatures ranging from 45 to 60° C.

The required work pressure for obtaining the desired conversion level was set at 250 psi (16.6 bar).

The resulting product was continuously fed into an agitated-tank-type reactor, at the same temperature, in order to allow the completion of the transesterification reaction.

The product obtained in the reactor outlet basically consisted in a biodiesel and glycerin blend, with 99.95% by weight of triglyceride conversion, compared to the triglyceride weight in the cavitation reactor inlet.

Step 5: This separation step of glycerin from biodiesel was carried out in a centrifuge, though decantation columns or both processes sequentially can be likewise used. Step 6: In this step, the glycerin-free biodiesel was processed using BD10 resins. Unlike conventional processes, when using the BD/10 resin, it is not necessary to previously disalcoholized the biodiesel to be purified, since the resin improves its effectiveness with up to 2% methanol. This is possible due to the amount of methanol used in this transesterification process, which allows using up to 30% less of methanol than conventional processes.

The amount of Ambersep BD10 resin needed for this biodiesel purification step is 3,300 kg. The column size shall be 3 times the empty space of the resin, 10.0 m3, with a coupling filter of 80 microns at the bottom. The column building material is carbon steel ST37. The resin change frequency depends on the impurities content, and is typically of 20 days.

The use of the BD/10 exchange resin for biodiesel purification, and the subsequent removal of soap, glycerin residues, methanol, and impurities replace washes with acidic water and neutral water used in conventional processes.

For this process, it was necessary to incorporate four columns, two of which worked continuously and were then replaced as the resin depleted in each of the columns by one which was in stand-by in order to recover the column having the depleted resin.

The column size was calculated depending on the flow per hour of the product to be purified. Out of the total volume of the column, 70% was left empty, since the resin, as it retains soaps, glycerin, and impurities, swallows until it completely fills the column volume.

Step 7: Finally, in this step, the substantially pure biodiesel was subject to conventional flash distillation to remove possible methanol residues.

By the end of the purification train, 999 L biodiesel were obtained, which complied with the international standard requirements.

Patents U.S. Pat. No. 7,582,784, U.S. Pat. No. 7,534,923, U.S. Pat. No. 7,550,614 and published patent applications US 20090275769, US 20090221844, US 20090156847, US 20090156844, US 20090048472, US 20080167485, US 20080114181, and US 20080015375 represent the resin, separation, and purification technologies which have been used for performing this experimental example. 

1. A continuous process for producing biodiesel fuel with a conversion of triglycerides in biodiesel above 99.90% by weight in the transesterification step, wherein it comprises the following steps: a) providing a triglyceride source at a triglyceride concentration above 99.0% by weight; b) subjecting triglycerides to a transesterification reaction with methanol, ethanol, or the mixtures thereof, at a molar ratio of triglycerides:alcohol 1:3-3.5, in the presence of 0.7 to 1.0% by weight of sodium or potassium hydroxide as catalyst, under ultrasonic cavitation conditions in an ultrasonic cavitation reactor with 2 to 8 serial cavitation cells, at a temperature of 45 to 60° C., at a pressure of 1.5 to 2 MPa (15-20 atm) and during a period of time of 15 to 30 seconds; c) recovering the product of step b) and subjecting it to mechanical agitation operation, at a temperature of 45 to 60° C., for a maximum of 10 minutes, up to the completion of the transesterification reaction, thus obtaining a conversion of triglycerides in biodiesel above 99.90% by weight; d) decanting and/or centrifuging the product resulting from step c) to remove glycerin; e) purifying biodiesel obtained in step d) by exchange resins; and f) removing by distillation methanol and water from the product of step e) and recovering the biodiesel fuel thus obtained.
 2. A process according to claim 1, wherein the triglyceride source is a new or used vegetable oil.
 3. A process according to claim 1, wherein the triglyceride source is a new or used animal fat.
 4. A process according to claim 1, wherein in the transesterification step, 0.95% by weight of sodium hydroxide is used, temperature is 50° C., and pressure is 16.6 atm.
 5. A process according to claim 1, wherein the product recovered from step b) is subject to mechanical agitation operation, at a temperature of 50° C., for a maximum of 10 minutes up to the completion of the transesterification reaction. 